Laser deposition of crystalline boron nitride films

ABSTRACT

Thin films of boron nitride are grown on single crystal silicon substrates using laser deposition techniques. The films are characterized by essentially a single crystal throughout and having a cubic structure which is in epitaxial registry with the underlying silicon substrate.

This is a continuation of U.S. Pat. application Ser. No. 07/446758 filedon 06-DEC-89 now abandoned.

This invention relates to the formation of thin films of boron nitrideIn particular, this invention relates to a method for forming thin filmsof single crystal, cubic phase boron nitride epitaxially oriented upon asilicon substrate using laser ablation techniques.

BACKGROUND OF THE INVENTION

Boron nitride (BN) is a most interesting III-IV compound from both thepractical and scientific viewpoints. Boron nitride is characterized bythree different crystal structures: hexagonal (hBN), wurtzite (wBN) andcubic zincblende (zBN). It is interesting that the physical propertiesof the boron nitride phase characterized by the cubic zincblende crystalstructure are comparable to diamond in that the cubic zincblende boronnitride and diamond have low densities, extremely high thermalconductivities, and large resistivities. In addition, the cubiczincblende boron nitride and diamond have similar tribologicalproperties and are relatively inert chemically.

There has been much research directed toward the growth of diamond thinfilms for various purposes. Therefore significant effort has also beendirected to growing these cubic zincblende boron nitride films for thesame purposes. This is not only because of the similarities between thetwo materials in their electrical, thermal and tribological properties,but because the cubic zincblende boron nitride may also prove to be anattractive substrate for subsequent diamond growth due to the smallmismatch in crystal lattice constants. As with the diamond films,previous attempts at depositing cubic boron nitride films have failed toproduce the desired homogeneous, single-crystal, and epitaxiallyoriented films.

Pulsed laser deposition of thin films has recently been demonstrated tobe a useful technique for preparation of thin films of a wide variety ofmaterials including polymers, semiconductors, superconductors, andnonlinear dielectrics. Typically, when utilizing laser depositiontechniques, a substrate of appropriate material is maintained at anelevated temperature opposite to a target having a composition the sameor similar to the desired thin film. A focused pulsed laser beam,usually from an excimer laser source, is incident on the target at anangle of approximately 45°. The deposition is generally performed in avacuum or other appropriate atmosphere such as flowing oxygen in thecase of the copper oxide superconductors.

Advantages of the laser deposition method over other depositiontechniques, such as evaporation, include a faster deposition rate, therequirement for only a single target, and the ability to depositmaterials possessing high boiling point temperatures, such as refractorymaterials. Advantages over sputtering deposition methods also includethe requirement of only a single target, as well as the preservation ofmaterial composition from the target to the film. It is thereforeadvantageous to use laser deposition techniques for the formation ofthin films of materials. In particular it would appear to beadvantageous to use these laser deposition methods for the formation ofthin films of materials such as boron nitride.

In summary, it is desirable to provide cubic boron nitride thin films,and particularly, to provide a method for forming these thin films,wherein the resulting thin films of cubic boron nitride are essentiallysingle-crystal, homogeneous and epitaxially oriented with the underlyingsubstrate.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide thin films of boronnitride.

It is a further object of this invention to provide a method for formingthese thin films of boron nitride using laser deposition techniques.

Lastly, it is still a further object of this invention that these thinfilms of boron nitride be characterized by a homogeneous single crystal,cubic structure, which is epitaxially aligned with an underlyingsubstrate.

In accordance with a preferred embodiment of this invention, these andother objects and advantages are accomplished as follows.

We are the first to grow thin films of boron nitride on single-crystalsilicon substrates using laser deposition techniques, wherein the filmsare characterized by being essentially single-crystal and having a cubicstructure which is in registry with the underlying silicon substrate.

This was accomplished by first providing a single-crystal siliconsubstrate oriented throughout along its [100] crystallographic axis. Aboron nitride target containing polycrystalline, hexagonally oriented,pyrolitic boron nitride was located opposite from the single-crystalsilicon substrate within a stainless steel 6-way cross chamber which wasevacuated to a pressure of about 3×10⁻⁴ Torr. The n-type single crystalsilicon substrate was heated to approximately 400° C. and maintainedthere during deposition. A KrF excimer laser operating at approximately248 nanometers and approximately 10 pulses per second was used as theablating beam. The laser ablation of the boron nitride target wasconducted in ultra high purity nitrogen gas, and at various laserfluences ranging from about 1.5 to 5.2 J/cm².

The deposited, boron nitride films were examined using characterizationprobes of transmission electron microscopy, scanning electronmiscroscopy, optical microscopy, and electron probe microanalysis. Theboron nitride films were determined to be essentially characterized by asingle-crystal, cubic structure which is in registry with the underlyingsilicon substrate.

Other objects and advantages of this invention will be betterappreciated from the detailed description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will become moreapparent from the following description taken in conjunction with theaccompanying drawing wherein:

FIG. 1 schematically illustrates the preferred laser ablation setup inaccordance with this invention for formation of thin films of singlecrystal, cubic boron nitride epitaxially oriented upon a siliconsubstrate, and

FIG. 2 is the transmission electron microscopy diffraction pattern for arepresentative sample of the thin films of single crystal, cubic boronnitride formed in accordance with the setup of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We are the first to provide a method for growing thin films of singlecrystal, cubic boron nitride, which are epitaxially oriented upon anunderlying silicon substrate.

As shown schematically in accompanying FIG. 1, a single-crystal siliconsubstrate 10 oriented throughout along its [100] crystallographic axisis provided. A boron nitride target 12 containing polycrystalline,hexagonally oriented, pyrolitic boron nitride was provided on a rotatingturntable 20 and located about 4 cm from the single-crystal siliconsubstrate 10 within a stainless steel 6-way cross chamber 14 which wasevacuated to a pressure of about 3×10⁻⁴ Torr. The n-type single crystalsilicon substrate 10 was heated by a heater 16 to approximately 400° C.and maintained there during the ablation and deposition process. Thefilms produced at 400° C. exhibited the desired cubic, epitaxiallyoriented boron nitride, however there was some signs of oxygenimpurities. Films have also been formed by heating the substrate to 675°C. during ablation and deposition. These films exhibit significantlyless impurities, however the desired cubic structure has not beenconfirmed. A KrF excimer laser source (the laser beam depicted as 18)operating at a wavelength of approximately 248 nanometers and afrequency of approximately 10 pulses per second was used as the ablatingbeam. The laser beam 18 emitted from the excimer laser source outsidethe chamber 14, passed through a transparent window 22 prior toincidence upon the target 12. The depositions were conducted in ultrahigh purity nitrogen gas and at various laser fluences ranging fromabout 1.5 to 5.2 J/cm².

More specifically, prior to the laser deposition, a polished n-type,single crystal, silicon wafer oriented along its [100] crystallographicaxis was provided. The wafer had a thin naturally occurring oxide layeron its surface believed to be on the order of several tens of Angstromsthick. The wafer was cut into approximately 1 square centimeter pieces,ultrasonically cleaned and fastened to a substrate holder. The siliconsubstrate was attached to the heater within the chamber and heated toapproximately 400° C. This temperature was maintained during thedeposition.

The laser depositions were performed in a stainless steel 6-way crosschamber which was evacuated by a turbomolecular pump to approximately3×10⁻⁴ Torr pressure. The target, a piece of polycrystalline, highoriented, hexagonal boron nitride commercially available from UnionCarbide, was rotated during the laser ablation to prevent excessivecratering within the target from the laser/target interaction.Unoriented, hexagonal BN target (Union Carbide commercial grades HBC orHBR) may also be used. A KrF excimer laser operating at a wavelength ofapproximately 248 nanometers and a frequency of approximately 10 pulsesper second, was used as the ablating beam. The target to substrateseparation was approximately 4 centimeters, but may vary between about2.5 centimeters and greater than 4 centimeters depending upon theoperating parameters of the laser.

The laser ablations of the boron nitride target were conducted withinthe evacuated chamber in the presence of ultra high purity, i.e.,approximately 99.99995%, nitrogen gas flowing at 10 sccm. For thepresent chamber geometry, this resulted in an ambient pressure ofapproximately 45 mTorr at the deposition surface of the siliconsubstrate. The presence of the nitrogen gas served to limit the size ofthe plasma plume generated by the interaction between the laser and thetarget, and to make the nitrogen concentration of the films nearlystoichiometric with the boron. We have also deposited using our methodin NH₃ gas to try to increase the nitrogen content. We believe the filmsare also cubic, epitaxial boron nitride. After the deposition of theboron nitride onto the silicon substrate, the film and substrate werecooled in flowing nitrogen to room temperature.

The laser depositions of boron nitride were conducted at various laserfluences ranging from approximately 1.5 to 5.2 J/cm². In a separateexperiment, the minimum threshold fluence for ablation of the hexagonalboron nitride target was determined to be approximately 0.31 to 0.34J/cm². Therefore, the laser depositions occurred at fluencessignificantly greater than the minimum threshold required.

The thickness of the boron nitride thin films as determined by stylusprofilometry, were found to vary linearly with laser fluence. Thethickness was also found to be linearly dependent on the number of laserpulses. At a laser fluence of approximately 3.9 J/cm², an averagedeposition rate of approximately 0.182 Angstroms per pulse was measured.For a 12,000 pulse run and a laser fluence of approximately 1.5 J/cm², afilm was produced having a thickness of approximately 176 nanometers.

In the films deposited in accordance with this invention, it wasobserved that the boron nitride/silicon interface had a blue appearance.This indicates that the films form an antireflective coating absorbingin the yellow-green region of the visible spectrum. The film thickness(d), index of refraction (n), and absorbed wavelength (λ) are related toeach other for antireflective coatings, through the equation

    nd=(m-1/2)x(λ/2)

where m is the interference order integer. For λ equal to 550 nanometers(yellow-green light), and a film thickness, d, equal to approximately176 nanometers, we find that n is equal to (m-1/2)×1.56. Clearly m equalto 1 is not the interference order since n can never be less than unity.However, with m equal to 2, n is equal to approximately 2.34, which isconsistent with that of cubic boron nitride at 2.1 and that of diamondat 2.42.

The film morphology was investigated with optical and scanning electronmicroscopies. To the resolution limit of the scanning electronmicroscopy instrument, approximately 100 Angstroms, no evidence of grainboundaries was observed. This implies that either the film is a singlecrystal, or that the film is amorphous possessing little or nocrystallinity. The latter hypothesis was excluded as a result of oursubsequent transmission electron microscopy research. The opticalmicroscope provided evidence that the film was of uniform thickness andnearly transparent. Micron sized particulates of undetermined originwere also observed to be sparsely distributed across the film surface,however, it is believed that these particulates can be eliminated withfurther development and testing of the operating parameters.

The relative atomic composition of the boron nitride thin films preparedin accordance with this invention were determined with electron probemicroanalysis employing the ZAF analytical technique. It was found thatthe resulting film is slightly nonstoichiometric with approximately 57%boron and 41% nitrogen, and with a plus or minus 10% relative error. Inaddition, trace impurities of carbon and oxygen, less than approximately1% each were observed.

The combination of low X-ray scattering intensities for boron nitrideand a 176 cm film thickness makes characterization using conventionalX-ray diffraction difficult. However, initial patterns on the resultingthin films of boron nitride showed indications of a crystalline cubicphase. This finding was confirmed through transmission electronmicroscopy, which is especially well suited to the aforementionedproblems. It was determined that the resulting thin films werecharacterized by a cubic structure. It is believed that the impingementof the laser on the hexagonally oriented boron nitride target results indissociation of the boron nitride compound, thereby permitting thevaporized boron and nitrogen atoms to deposit on and align themselveswith the cubic silicon substrate.

In order to use transmission electron microscopy, the substrates had tobe slowly etched in order to obtain boron nitride films having athickness of approximately 50 to 100 Angstroms. A dilute acid etchcomprising hydrofluoric acid and nitric acid in water was used to reducethe thickness of the silicon to less than 1 micron. The boronnitride/silicon substrate was then ion-milled thereby producing regionsof extremely thin boron nitride.

The transmission electron microscopy diffraction pattern of arepresentative region showed a diffraction pattern of a cubic crystalwith a lattice constant of approximately 3.8 Angstroms. However theaccepted lattice constant of cubic zincblende boron nitride powder isapproximately 3.6 Angstroms. Thus the thin film lattice for the cubicboron nitride formed in accordance with this invention is expanded byabout 5% from the accepted cubic zincblende boron nitride powderlattice. The 3.8 Angstrom lattice constant of our boron nitride filmclosely resembles a dense, cubic form of boron nitride called"shock-wave compressed".

As shown in FIG. 2, the transmission electron microscopy diffractionpatterns for the resulting cubic boron nitride films formed upon thesilicon substrate contain a superposition of a [211] plane of siliconand a (100) plane of cubic boron nitride. This diffraction pattern showsthat the [022] direction of silicon is parallel to the (020) directionof the cubic boron nitride films, and that the [111] direction ofsilicon is almost parallel to the (002) direction of cubic boronnitride. Therefore, there is a preferred orientation for the cubic boronnitride films with respect to the underlying silicon substrate, with the(020) direction of cubic boron nitride parallel to the [022] directionof silicon. This is considered to be substantial evidence that the cubicboron nitride film is epitaxially aligned with the silicon substrate.

The 3.8 Angstrom lattice constant of the cubic boron nitride film isparticularly well suited to the silicon substrate. Since silicon has adiamond-like crystallographic structure with a lattice constant ofapproximately 5.42 Angstroms, two cubic boron nitride lattice constantscan fit across the [100] diagonal of the silicon lattice, i.e., 3.8Angstroms equals 5.42 Angstoms divided by 2^(1/2). In thisconfiguration, either the boron or the nitrogen atoms will reside overthe silicon atoms.

We have also deposited using this method boron nitride on single crystalsilicon substrates oriented along the [110] crystallographic plane andbelieve that the transmission electron microscopy results will showcubic, epitaxial boron nitride. We have deposited on single crystalsilicon oriented along the [111] plane and this results in hexagonallyoriented boron nitride. Cubic, epitaxially aligned silicon has also beenobtained by our methods using 193 nanometer radiation from an ArFexcimer laser at the same fluences described above, but at a photonenergy of about 6.2 electron volts.

In general, for every known and potential application considered fordiamond films, cubic boron nitride may be an appropriate substitute.Since cubic boron nitride is an excellent insulator which can be grownepitaxially on silicon and presumably has a high thermal conductivitysimilar to the cubic zincblende boron nitride, cubic boron nitride willhave numerous uses as an insulating, thermally conductive barrier layerfor silicon-based microelectronic devices. Further, the cubic boronnitride should be appropriate for many wear resistant applications,since the cubic zincblende boron nitride is second in hardness only todiamond.

This invention readily provides a method for forming single-crystal,epitaxially aligned thin films of cubic boron nitride on single-crystalsilicon substrates. While our invention has been described in terms ofpreferred embodiments, it is apparent that other forms of the devicecould be adopted by one skilled in the art, such as by substitutingmaterials or processing components, as well as by varying the processingparameters. Accordingly the scope of our invention is to be limited onlyby the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for formingthin films of boron nitride comprising the following steps:providing asilicon substrate oriented essentially throughout along its [100]crystallographic axis and heating said substrate to approximately 400°C.; providing a boron nitride target comprised essentially ofhexagonally oriented pyrolitic boron nitride, said boron nitride targetbeing disposed in proximity with said silicon substrate; inducingvaporization of said boron nitride target using a laser operating atappropriate parameters, such that said vaporized boron nitride depositsonto a surface of said silicon substrate to form a thin film layer ofboron nitride on said silicon surface, said thin film layer of boronnitride being characterized by a single crystal, cubic structure andcrystallographically aligned with said silicon substrate.
 2. A methodfor forming thin films of boron nitride as recited in claim 1 whereinsaid laser is a KrF excimer laser source which operates at a wavelengthof approximately 248 nanometers.